1. Field of the Invention
The present invention relates to a method for manufacturing a porous body fit for use at a high temperature.
2. Description of the Related Art
Great hopes are set on a ceramic porous body which is used as a plane combustion porous board, a heat transfer sensing element and a dust removing filter. The plane combustion porous board is used at a high temperature of 1000.degree. C. or more. The heat transfer sensing element converts heat obtained by convective heat transfer to radiative heat. The dust removing filter is used for high-temperature dust containing gas. An alumina porous body is often applied for the aforementioned use. A Japanese Patent Publication Laid Open No. 167290/89 discloses that silicon carbide or silicon nitride is formed on the surfaces of a preform of carbon fibers. It has been difficult to apply the prior art ceramic porous body to the use wherein the ceramic body is repeatedly heated and cooled in an oxidizing atmosphere at temperatures between a temperature of more than 1000.degree. C. and normal temperature. That is, since the thermal expansion coefficient of the alumina porous body is large and the heat conductivity of the alumina porous body is small, the alumina porous body is liable to crack at the steps of heating and cooling. In consequence, the alumina porous body is not fit for use for a long period of time. In the case of carbon material coated with silicon carbide or silicon nitride, the thermal expansion coefficient of the coating layer is larger than that of the base material, the coating layer is split off at the step of repeated heating and cooling, and carbon in the base material can be exposed and combusted.